U.S. patent number 4,622,516 [Application Number 06/505,047] was granted by the patent office on 1986-11-11 for magnetic tachometer for disk drives.
This patent grant is currently assigned to Digital Equipment Corporation. Invention is credited to Patrick L. Hearn, Shyam C. Parikh, Charles M. Riggle, Kenneth F. Veseskis.
United States Patent |
4,622,516 |
Hearn , et al. |
November 11, 1986 |
Magnetic tachometer for disk drives
Abstract
A magnetic tachometer for generating a signal as a function of
the velocity of a transducer positioning arm in a disk drive. The
tachometer is formed by a pair of fixed parallel coils separated by
a distance sufficient to allow a magnet, attached to a
counterbalance portion of the rotary positioning arm, to move
therebetween and thus to generate a voltage as a function of the
velocity of the magnet. The coils are each wound over a thin
armature member which is saturated by the magnet's magnetic field
and they are differentially coupled to produce common mode
rejection.
Inventors: |
Hearn; Patrick L. (Acton,
MA), Riggle; Charles M. (Colorado Springs, CO), Parikh;
Shyam C. (Stow, MA), Veseskis; Kenneth F. (Hudson,
MA) |
Assignee: |
Digital Equipment Corporation
(Maynard, MA)
|
Family
ID: |
24008781 |
Appl.
No.: |
06/505,047 |
Filed: |
June 16, 1983 |
Current U.S.
Class: |
324/163;
G9B/5.187 |
Current CPC
Class: |
G01P
3/465 (20130101); G11B 5/5521 (20130101) |
Current International
Class: |
G11B
5/55 (20060101); G01P 3/42 (20060101); G01P
3/46 (20060101); G01P 003/46 () |
Field of
Search: |
;324/163,164,166,173,174,207 ;360/97,86,104,105,106,109
;310/36,38 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Harvey; Jack B.
Attorney, Agent or Firm: Cesari and McKenna
Claims
What is claimed is:
1. An arm assembly for use in a disk drive comprising:
A. arm means for supporting a magnetic transducer for writing data
onto and reading data from a disk surface, said arm means being
pivotal in a pivot plane about a pivot point, said arm means
further supporting a permanent magnet means a selected distance
from said pivot point said permanent magnet means generating a
magnetic flux;
B. motor means attached to said arm means for pivoting said arm
mean through a selected maximum arc; and
C. substantially longitudinal electrical coil means having two ends
and an axis parallel to said pivot plane, said coil means being
positioned to intercept the magnetic flux of said permanent magnet
means as said arm means is pivoted through said arc so that the
movement of said magnet impresses an electrical potential
difference between the ends of said coil means, the angular
velocity of said arm means as it is being pivoted by said motor
means being related to the impressed potential difference.
2. An arm assembly as defined in claim 1 wherein said coil means
comprises a coil disposed about a coil support comprising a
high-permeability elongated member, the coil means being positioned
such that the magnetic flux generated by said magnet substantially
saturates said coil support.
3. An arm assembly as defined in claim 1 wherein said coil means
includes a pair of substantially longitudinal wire coils each
having an axis parallel to and on opposing sides of pivot plane of
said arm means, and further including spacer means for separating
said coils by a selected distance to form a slot, with the magnet
means moving in the slot.
4. An arm assembly as defined in claim 3 wherein said coils are
electrically interconnected so as to provide rejection of common
mode signals.
5. An arm assembly as defined in claim 4 wherein each of said coils
is disposed about a coil support comprising a thin armature means
having an insulating layer, each of said coils being disposed about
the respective insulating layer.
6. An arm assembly as defined in claim 1 wherein said permanent
magnet means is supported by said arm means so that its
magnetization is perpendicular to the pivot plane of said arm
means.
Description
BACKGROUND OF THE INVENTION
This invention relates to tachometers for monitoring the velocity
of the read/write heads in a disk drive, and more specifically to
tachometers for monitoring the velocity of a rotary actuator arm of
a disk drive.
In general, a closed loop disk drive uses a servo system to
accurately position the read/write head at a requested track. In
some applications the servo system uses a dedicated disk surface on
which servo information is written. In these applications no
tachometer is needed, since the dedicated servo surface provides
continuous feedback information. Obviously such systems have the
disadvantage of having one less disk surface available for data
storage, and for systems having one or two disks this represents a
substantial storage loss. To alleviate this data storage loss while
maintaining the accuracy of the servo system, some applications use
a tachometer, coupled to the head actuator, to relay velocity
information to the servo system. The servo system then uses the
head velocity signal as the continuous servo signal, this
information being supplemented with fine positioning information
stored on a small portion of each sector of each track and normally
referred to as embedded servo information.
Generally, a disk drive may use either linear or rotary actuator
positioning means. In the linear case, the actuator arm, and
therefore the head, is moved linearly along a radius of the
rotating disk, while in the rotary case, the actuator arm rotates
along an axis parallel to the disk spindle at a point close to the
outside rim of the disk. In either case. a suitable tachometer must
be used.
Known linear tachometer designs for disk drive applications are not
directly applicable to rotary actuator designs. Generally magnetic
tachometer designs are very susceptible to stray magnetic fields,
and optical tachometers using a glass scale are too expensive.
The available rotary tachometers are not designed for disk drive
applications. They range from simple generators that measure the
speed of the rotating shaft to more sophisticated optical decoders.
Unfortunately they are either not sensitive enough, due in part to
the lack of noise rejection and adequate bandwidth, or are too
expensive for disk drive applications. Cost is an important factor
in the manufacture of reasonably priced disk drives, thus an
inexpensive tachometer is needed, provided that the required
bandwith and noise rejection can be achieved.
SUMMARY OF THE INVENTION
The present invention provides for an inexpensive magnetic
tachometer which produces a bandwidth and noise rejection
particularly suitable for disk drive applications.
An elongated actuator arm, having first and second end portions
opposite each other, carries an electromagnetic transducer on the
first end portion, the transducer being adapted to be used over a
rotating magnetic disk. A magnet is disposed on the second end
portion. A substantially longitudinal conductive coil is disposed
adjacent the magnet at a position such that the path defined by the
magnet throughout the range of motion of the actuator arm remains
within an area defined by the projection of the coil's outer
dimensions in the plane of the path of the magnet.
In the preferred embodiment, two longitudinal coils are disposed
parallel to each other to define an elongated region in which a
magnet, positioned on the counter-balance end of a rotary actuator
arm, is free to move as the arm rotates through selected positions.
The coil assembly is rigidly supported on a frame mounted on the
casing of the servo motor. The structure of the coil assembly helps
to reduce costs, since the two coils can be simultaneously wound
over a single elongated armature/filler subassembly. This
subassembly is then snapped in two and bent over spacers to form a
rigid coil assembly. A flexible conductor is used to provide a
connection to the coils. The armature is a thin, elongated,
low-loss member and is magnetically saturated by the magnetic field
available from the magnet. The two coils are differentially coupled
to provide common mode rejection. The saturation of the magnetic
circuit, the differentially coupled coils and the direct coupling
of the magnet to the actuator arm contribute to provide a magnetic
tachometer having sufficiently large bandwidth and noise rejection
for disk drive applications.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the present invention may be obtained
from the accompanying description used in conjunction with the
drawings in which:
FIG. 1 is an exploded view of the tachometer and arm assembly for
the present invention;
FIGS. 2 A-B are two similar top views of the tachometer coil
assembly before and after winding, respectively;
FIG. 2C shows a cross-section of the assembly of FIG. 2B;
FIG. 2D shows a side view of the assembled coil subassembly;
FIG. 3A is a diagrammatic view showing the magnetic and electric
circuits for the tachometer assembly of the present invention;
FIG. 3B is a block diagram showing the servo system used to
position the actuator; and
FIG. 3C shows the equivalent circuit for the present tachometer
assembly and the electrical circuit used to generate a single
velocity signal.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown the tachometer assembly to
the present invention. Tachometer assembly 10 will be described in
more detail below, suffice it to say for now that is comprises two
coils 12 and 14 disposed parallel to each other to define an
elongated region therebetween. Within this region there is located
a disk-shaped magnet 16 which has the opposite polarity poles on
the two opposite surfaces and is imbedded in the counter weight
portion 22 of arm 20. Attached to the opposite end of arm 20 is
read/write head 26 which is used to read or write magnetically
encoded information on one surface of a magnetic disk (not shown).
The read/write head 26 has a set of wires 28 running from its
magnetic core and coil assembly along the elongated portion 24 of
arm 20 for conventional connection to suitable read/write
circuitry. Arm 20 is attached to the shaft 32 of rotary servo motor
30. Servo motor 30 rotates the arm 20 through a predetermined arc
and therefore positions head 26 at a location corresponding to the
desired track on the disk.
Tachometer assembly 10 is fastened to and supported by block 40
which is secured to plate 42. Plate 42 is secured to the casing of
servo motor 30 and is used, together with block 40, to support
tachometer assembly 10 at an appropriate fixed position with
respect to the movable arm. The appropriate position is such that
the projected region between the two coils encloses arm magnet 16
throughout the permitted movement range of arm 20.
Referring how also to FIGS. 2 A-D, the tachometer assembly 10 will
be discussed in more detail. Each of the two coils 12 and 14 is
wound on a corresponding coil support formed by armature plate 52
and filler 54. Armature plate 52 is a thin U-shaped elongated plate
of electrical steel. The bent edges forming the U provide the
required amount of stiffness to the structure. The area within the
U is filled with a slab of plastic filler 54 to support the coils.
The two coils 12 and 14 are formed by winding insulated copper wire
over a central portion of armature plate 52/filler 54 assembly. To
further aid in the containment of the coils, plastic filler 54
includes two end shoulders 58 which extend up to the top surface of
the coil to provide an overall flush surface. The electrical
connection to the coils is made by using a flexible conductor cable
60. A first conductor is connected to pad 62 where the beginning of
the winding of coil 12 is soldered. A second conductor is connected
first to pad 64 where the end of the winding of coil 12 is
soldered. This second conductor then continues and is connected to
pad 66 where the beginning of the winding of coil 14 is also
soldered. The third conductor is coupled to pad 68 where the end of
the winding of coil 14 is also soldered. The coils 12 and 14 are
wound in the same direction with respect to the beginning of their
respective windings, and each end of the winding is brought back to
a pad, 68 and 64 respectively, adjacent the location of the
beginning of windings. The portion of flex cable 60 connected to
coil 14 is placed on a suitable longitudinal groove formed in
filler 54, and it is held in place by coil 12.
A preferred method of manufacture for tachometer assembly 10 uses a
one piece plastic insert 70 having two ends 54, as described above,
connected by a central breakaway portion 72. To each of the two
ends 54 there is bonded, by conventional means, armature piece 52.
Flex connector 60 is then connected to solder pads 62, 64, 66, and
68. To insulate the windings from the armature, a thin sheet of
mylar tape is applied over the armature. The coils 12 and 16 are
then wound simultaneously and in the same direction starting from
solder pads 62 and 66, respectively. At the end of the winding step
the coil wire is brought back and soldered to pads 64 and 68
respectively. A further layer of insulation is placed over the
coils, for example by using mylar shrink tubing. The wound assembly
is now bent causing the center portion 72 of plastic insert 70 to
breakaway at four break points 74. The two wound coil
subassemblies, now only connected by flex cable 60, are folded, and
four cylindrical spacers 76 are used to complete the tachometer
assembly. Each spacer 76 has reduced diameter hollow end portions
which fit into corresponding holes in armature plate 52 and hold
the two coils 12 and 14 at a predetermined separation. A conical
swaging tool is used to lock the spacers to the two armatures.
Referring now to FIG. 3A, there is shown a simplified diagram of
the magnet-coils interaction. Magnet 16 provides a predetermined
amount of magnetic flux, shown by representative magnetic flux
lines 17. The magnetic flux produced by magnet 16 is mostly
confined to the closed magnetic circuit depicted as composite
armature 18. Armature 18 is actually formed by the two armature 52
in combination with the four spacers 76 (see FIG. 2D). Each
armature plate is preferably formed of stock rolled steel and cut
so that the direction of mill rolling, i.e. the grain direction, is
along the longitudinal axis of the elongated armature plate, for
maximizing the permeability of the plate along this axis in order
to improve its saturation characteristics. Preferably, magnet 16 is
a rare-earth cobalt magnet. The relative values of the magnetic
field provided by magnet 16 and the physical dimension of armatures
52 are selected such that the armatures are magnetically saturated.
This has the advantage of reducing the sensitivity of the coils to
stray magnetic fields, i.e. fields other than that provided by
tachometer magnet 16. To further reduce immunity to noise, i.e.
stray fields, the two coils 12 and 14 are coupled together in a
bucking configuration, that is, for a magnetic field in a direction
along the longitudinal axis of the coils, the output of the two
coils will cancel. This is achieved, as shown in FIG. 3B, by
coupling the output of each coil to a differential amplifier 100
such that its common mode rejection cancels out the signals due to
stray fields.
Referring now to FIG. 3C, there is shown the electrical circuit for
the tachometer/differential amplifier subsystem. It can be seen
that the output voltage V.sub.o of differential amplifier 100 is as
follows;
where,
V.sub.t1 =voltage output of coil 12 due to tachometer magnet
V.sub.t2 =voltage output of coil 14 due to tachometer magnet
V.sub.cm1 =voltage output of coil 12 from stray field
V.sub.cm2 =voltage output of coil 14 from stray field
Thus, the advantage of using a bucking configuration for the two
coils is that, in addition to cancelling the noise output, the
desired tachometer output from each coil is added. As an aid in
reducing noise due to magnetic fields generated by servo motor 30,
a magnetic shielding cylindrical sleeve made of a high permeability
material may be used around the casing of servo motor 30, although
most of the flux lines produced by the servo motor are
perpendicular to the longitudinal axis of coils 12 and 14, and thus
do not induce a voltage therein.
Using Faraday's Law, it can be seen that the voltage V.sub.t
produced in each coil by the magnetic field of magnet 16 is the
line integral taken about the electromagnetic circuit,
which yields
where v.sub.m =velocity of the magnet, B.sub.m =remnant flux
density of magnet, A.sub.m =area of magnet, N=number of turns, and
b=length of coil. Since the magnet and head are attached to the
same pivot:
where .theta.=angular velocity, r.sub.m =radius of magnet, r.sub.h
=radius of head, and v.sub.h =velocity of head. Therefore,
which yields
In one embodiment, the various parameters yield 23 mV/(inch/sec)
for each coil. The gain of the system is designed to produce an
output voltage V.sub.o =0.2 V (inch/sec).
The path of the magnet within the region between the two coils is
actually arcuate, but for the small angular rotation required to
position head 26 between its two extreme operational positions this
path substantially approximates the longitudinal axis of the
tachometer assembly 10, and no correction is found necessary.
The shape of the cross-section of each coil is defined by
cross-section of the armature/filler subassembly, and in the
preferred embodiment it substantially resembles a very thin
rectangle. This has the advantage of providing a very compact and
rigid structure while effectively enclosing, in combination with
armature 52, substantially all the magnetic flux lines generated by
magnet 16.
Referring now back to FIG. 3B, there is shown a block diagram
illustrating the arm positioning subsystem of a disk drive.
Tachometer magnet 16, carried in the counter weight portion 22 of
actuator arm 20, is shown diagramatically being contained within
tachometer assembly 10. The output of tachometer assembly 10 is
coupled to differential amplifier 100 to produce a signal as a
function of the speed of read/write head 26, as explained above.
The speed signal thus produced is fed into servo circuit 110. The
signals from read/write head 26 are fed to read/write circuit 120,
where the embedded servo information is stripped off. The servo
data thus produced is fed into servo circuit 110. Servo circuit 110
uses the velocity signal, the embedded servo signals and a selected
track signal from a disk controller to generate a control signal
used by servo motor driver 130 to control the operation of servo
motor 140, which positions arm 20 at the selected track.
The servo circuit 110, which may be of conventional design uses the
velocity information to provide a higher servo loop bandwidth, to
provide the primary servo signal while the head is seeking the next
track, and to provide a secondary position signal. A position
signal is obtained in servo circuit 110 by integrating the velocity
signal. The velocity signal is also used to determine the arm's
position when loading the head, since when the head is not loaded
on the disk, and therefore not reading the recorded information, no
other servo information is available.
This concludes the description of the preferred embodiment.
Modifications to the preferred embodiment will also be apparent to
those skilled in the art without departing from the spirit and
scope of the present invention. Accordingly, it is intended that
this invention be not limited to the embodiments disclosed herein
except as defined by the appended claims.
* * * * *